1. Field of the Invention
This invention relates, in general, to resistors, and more particularly to the formation, composition, and use of an improved ternary intermetallic compound as a thin film resistor material on electronic devices, generally with semiconductor devices, and further, to improved semiconductor devices and circuits incorporating this resistor material.
2. Description of the Prior Art
Resistors are widely used in electronic devices to inhibit the flow of electric current. Frequently, resistors in thin film form are combined with semiconductor devices to make extremely compact, yet complex structures. The thin film resistors may be a part of an individual device, as for example, an emitter ballast resistor in a power transistor, or they may be used in connection with a multiplicity of semiconductor devices to form a more complex electrical function such as in a hybrid or integrated circuit. A resistive divider network in an analog-to-digital converter, or current limiting and load resistors in an emitter follower amplifier, are examples of applications wherein thin film resistors are used in complex hybrid and/or integrated circuits.
Film resistors are usually characterized in terms of their sheet resistivity and their temperature dependence. Sheet resistivity is expressed in resistance per unit area (e.g. ohms per square) and is equal to the bulk resistivity divided by the film thickness. Resistivity is a material property and is not dependent on the topology of a particular resistor. The resistance of a specific resistor is obtained by multiplying the sheet resistivity by the ratio of the resistor length to width.
For compact devices and circuits, especially complex integrated circuits (IC's), it is generally desired that film resistor materials have a sheet resistivity greater than 100 ohms per square, with 500 to 1500 ohms per square being a particularly convenient range for many applications. Examples of prior art film resistor materials and their typical ranges of sheet resistivities (expressed in ohms per square and given in parenthesis following each composition) are: Ni-Cr (40-400); Cr-Si (100-5000); Ta (100-1000); and Cr-Si0 (100-1000).
The temperature dependence of thin film resistors is described in terms of the temperature coefficient of resistivity (TCR) which reflects the slope of the resistivity versus temperature curve, that is, the fractional change in resistance per unit change in temperature. It is usually expressed in parts per million change per degree centigrade (ppm per .degree.C). The TCR may be positive or negative and may vary with temperature. Prior art film resistor materials typically have TCR's of the order of a few hundred to a few thousand parts per million per degree C., positive or negative, and varying with temperature. Both the resistivity and the TCR can be sensitive to the choice of material, method of preparation, substrate surface, ambient atmosphere, and annealing (heat treatment) subsequent to formation.
It is desired that resistor materials be readily prepared in controlled thicknesses and convenient resistivities, be easily patterned and dimensionally stable, be amendable to the formation thereon of low resistance, void free, and stable contacts, be compatible with other steps essential to the overall circuit or device manufacturing process, and have electrical characteristics which are stable over long periods of time. It is further desired that the TCR be controllable, that is, have a value which is substantially independent of temperature and which can be selected to have a predetermined positive, negative, or zero value. Zero TCR can generally be achieved only over a very limited temperature range, and usually in connection with a temperature dependent TCR. For example, Cr-Si films can have TCR's of 0.+-.50 ppm per .degree.C., but have been found to have a parabolic variation of resistivity with temperature. It is more convenient to have a TCR which is temperature independent, that is, where the resistivity is a linear function of temperature over the temperature range of interest for most electrical apparatus (e.g. -55.degree. to +125.degree. C.). Some materials, for example Cr-Si, react with or dissolve in commonly used contact metals, such as Al, producing thin spots or voids adjacent to the contacts, with a resulting loss of reliability. It is desirable to avoid this effect. The prior art film resistor materials, preparation methods, and structures do not give film resistors, as far as is known, having the above combination of desirable features.
Accordingly, it is an object of this invention to provide an improved resistor material for electrical circuits and devices.
It is a further object of this invention to provide an improved resistor material for electrical structures which can be readily prepared in convenient resistivities and thicknesses, which is easily patterned, which is dimensionally stable, which is amendable in stable low resistance electrical contacts, which is compatible with other devices or circuit processing steps and materials, which is stable over time and which has a controllable TCR that is substantially independent of temperature or is zero in the temperature range of interest.
It is an additional object of this invention to provide an improved resistor material for electrical devices wherein the TCR can be set to have substantially constant positive, negative, or zero values over a temperature range from -55.degree. to +125.degree. C.
It is a further object of this invention to provide improved semiconductor devices, hybrid or integrated circuits, and resistor chips having thereon improved thin film resistors of predetermined values.
It is an additional object of this invention to provide a resistor film material which does not give rise to voids or thin regions in contact with common contact or interconnect metals such as aluminum.
It is a still further object of this invention to provide processes for the fabrication of improved film resistor materials and resistor structures, and improved devices and circuits utilizing these materials and structures.